Some of them are fine. Alarak's are perhaps the best in my opinion: high risk, high reward. If you don't complete them then you're a lame duck. Complete them, especially early in the game, and you destroy people.

Some are terrible, however: looking at you, Thrall. That might just be because Chain Lightning feels like one of the worst abilities in the game though.

One of the worst quest talents I can think of is Zagara's lvl 7 talent, Bile Drop (it might even be the worst). That quest is in a practical sense impossible to complete. Thrall's quest talents, while a bit excessive in how plenty they are and how hard they are to complete, at least can be completed. I don't think I have ever been able to do that with Zagara's Bile Drop talent. Unless the enemy is literally AFKing, the odds of landing a hit is slim to none, simply because the projectiles can be dodged by people with even the slowest reaction times.